As the name implies a javax.ejb.Singleton is a session bean with a guarantee that there is at most one instance in the application.
What it gives that is completely missing in EJB 3.0 and prior versions is the ability to have an EJB that is notified when the application starts and notified when the application stops. So you can do all sorts of things that you previously could only do with a load-on-startup servlet. It also gives you a place to hold data that pertains to the entire application and all users using it, without the need for a static. Additionally, Singleton beans can be invoked by several threads at one time similar to a Servlet.
See the Singleton Beans page for a full description of the javax.ejb.Singleton api.
Here we see a bean that uses the Bean-Managed Concurrency option as well as the @Startup annotation which causes the bean to be instantiated by the container when the application starts. Singleton beans with @ConcurrencyManagement(BEAN) are responsible for their own thread-safety. The bean shown is a simple properties "registry" and provides a place where options could be set and retrieved by all beans in the application.
package org.superbiz.registry;
//START SNIPPET: code
import javax.annotation.PostConstruct;
import javax.annotation.PreDestroy;
import javax.ejb.ConcurrencyManagement;
import javax.ejb.Singleton;
import javax.ejb.Startup;
import java.util.Properties;
import static javax.ejb.ConcurrencyManagementType.BEAN;
@Singleton
@Startup
@ConcurrencyManagement(BEAN)
public class PropertyRegistry {
// Note the java.util.Properties object is a thread-safe
// collections that uses synchronization. If it didn't
// you would have to use some form of synchronization
// to ensure the PropertyRegistryBean is thread-safe.
private final Properties properties = new Properties();
// The @Startup method ensures that this method is
// called when the application starts up.
@PostConstruct
public void applicationStartup() {
properties.putAll(System.getProperties());
}
@PreDestroy
public void applicationShutdown() {
properties.clear();
}
public String getProperty(String key) {
return properties.getProperty(key);
}
public String setProperty(String key, String value) {
return (String) properties.setProperty(key, value);
}
public String removeProperty(String key) {
return (String) properties.remove(key);
}
}
Here we see a bean that uses the Container-Managed Concurrency option, the default. With @ConcurrencyManagement(CONTAINER) the container controls whether multi-threaded access should be allowed to the bean (@Lock(READ)) or if single-threaded access should be enforced (@Lock(WRITE)).
package org.superbiz.registry;
import javax.ejb.Lock;
import javax.ejb.Singleton;
import java.util.ArrayList;
import java.util.Collection;
import java.util.HashMap;
import java.util.Map;
import static javax.ejb.LockType.READ;
import static javax.ejb.LockType.WRITE;
@Singleton
@Lock(READ)
public class ComponentRegistry {
private final Map<Class, Object> components = new HashMap<Class, Object>();
public <T> T getComponent(Class<T> type) {
return (T) components.get(type);
}
public Collection<?> getComponents() {
return new ArrayList(components.values());
}
@Lock(WRITE)
public <T> T setComponent(Class<T> type, T value) {
return (T) components.put(type, value);
}
@Lock(WRITE)
public <T> T removeComponent(Class<T> type) {
return (T) components.remove(type);
}
}
Unless specified explicitly on the bean class or a method, the default @Lock value is @Lock(WRITE). The code above uses the @Lock(READ) annotation on bean class to change the default so that multi-threaded access is granted by default. We then only need to apply the @Lock(WRITE) annotation to the methods that modify the state of the bean.
Essentially @Lock(READ) allows multithreaded access to the Singleton bean instance unless someone is invoking an @Lock(WRITE) method. With @Lock(WRITE), the thread invoking the bean will be guaranteed to have exclusive access to the Singleton bean instance for the duration of its invocation. This combination allows the bean instance to use data types that are not normally thread safe. Great care must still be used, though.
In the example we see ComponentRegistryBean using a java.util.HashMap which is not synchronized. To make this ok we do three things:
@Lock(WRITE) on the methods that mutate the map such as the put() and remove() methods.@Lock(READ) on the get() and values() methods as they do not change the map state and are guaranteed not to be called at the same as any of the @Lock(WRITE) methods, so we know the state of the HashMap is no being mutated and therefore safe for reading.The end result is that the threading model for this bean will switch from multi-threaded access to single-threaded access dynamically as needed, depending on the method being invoked. This gives Singletons a bit of an advantage over Servlets for processing multi-threaded requests.
See the Singleton Beans page for more advanced details on Container-Managed Concurrency.
package org.superbiz.registry;
import junit.framework.TestCase;
import javax.ejb.embeddable.EJBContainer;
import javax.naming.Context;
import java.net.URI;
import java.util.Date;
//START SNIPPET: code
public class ComponentRegistryTest extends TestCase {
public void test() throws Exception {
final Context context = EJBContainer.createEJBContainer().getContext();
// Both references below will point to the exact same instance
ComponentRegistry one = (ComponentRegistry) context.lookup("java:global/simple-singleton/ComponentRegistry");
ComponentRegistry two = (ComponentRegistry) context.lookup("java:global/simple-singleton/ComponentRegistry");
// Let's prove both references point to the same instance
// Set a URL into 'one' and retrieve it from 'two'
URI expectedUri = new URI("foo://bar/baz");
one.setComponent(URI.class, expectedUri);
URI actualUri = two.getComponent(URI.class);
assertSame(expectedUri, actualUri);
// Set a Date into 'two' and retrieve it from 'one'
Date expectedDate = new Date();
two.setComponent(Date.class, expectedDate);
Date actualDate = one.getComponent(Date.class);
assertSame(expectedDate, actualDate);
}
}
package org.superbiz.registry;
import junit.framework.TestCase;
import javax.ejb.embeddable.EJBContainer;
import javax.naming.Context;
public class PropertiesRegistryTest extends TestCase {
public void test() throws Exception {
Context context = EJBContainer.createEJBContainer().getContext();
PropertyRegistry one = (PropertyRegistry) context.lookup("java:global/simple-singleton/PropertyRegistry");
PropertyRegistry two = (PropertyRegistry) context.lookup("java:global/simple-singleton/PropertyRegistry");
one.setProperty("url", "http://superbiz.org");
String url = two.getProperty("url");
assertEquals("http://superbiz.org", url);
two.setProperty("version", "1.0.5");
String version = one.getProperty("version");
assertEquals("1.0.5", version);
}
}
Running the example is fairly simple. In the "simple-singleton" directory run:
$ mvn clean install
Which should create output like the following.
-------------------------------------------------------
T E S T S
-------------------------------------------------------
Running org.superbiz.registry.ComponentRegistryTest
Apache OpenEJB 4.0.0-beta-1 build: 20111002-04:06
http://openejb.apache.org/
INFO - openejb.home = /Users/dblevins/examples/simple-singleton
INFO - openejb.base = /Users/dblevins/examples/simple-singleton
INFO - Using 'javax.ejb.embeddable.EJBContainer=true'
INFO - Configuring Service(id=Default Security Service, type=SecurityService, provider-id=Default Security Service)
INFO - Configuring Service(id=Default Transaction Manager, type=TransactionManager, provider-id=Default Transaction Manager)
INFO - Found EjbModule in classpath: /Users/dblevins/examples/simple-singleton/target/classes
INFO - Beginning load: /Users/dblevins/examples/simple-singleton/target/classes
INFO - Configuring enterprise application: /Users/dblevins/examples/simple-singleton
INFO - Configuring Service(id=Default Singleton Container, type=Container, provider-id=Default Singleton Container)
INFO - Auto-creating a container for bean PropertyRegistry: Container(type=SINGLETON, id=Default Singleton Container)
INFO - Configuring Service(id=Default Managed Container, type=Container, provider-id=Default Managed Container)
INFO - Auto-creating a container for bean org.superbiz.registry.ComponentRegistryTest: Container(type=MANAGED, id=Default Managed Container)
INFO - Enterprise application "/Users/dblevins/examples/simple-singleton" loaded.
INFO - Assembling app: /Users/dblevins/examples/simple-singleton
INFO - Jndi(name="java:global/simple-singleton/PropertyRegistry!org.superbiz.registry.PropertyRegistry")
INFO - Jndi(name="java:global/simple-singleton/PropertyRegistry")
INFO - Jndi(name="java:global/simple-singleton/ComponentRegistry!org.superbiz.registry.ComponentRegistry")
INFO - Jndi(name="java:global/simple-singleton/ComponentRegistry")
INFO - Jndi(name="java:global/EjbModule453799164/org.superbiz.registry.ComponentRegistryTest!org.superbiz.registry.ComponentRegistryTest")
INFO - Jndi(name="java:global/EjbModule453799164/org.superbiz.registry.ComponentRegistryTest")
INFO - Created Ejb(deployment-id=org.superbiz.registry.ComponentRegistryTest, ejb-name=org.superbiz.registry.ComponentRegistryTest, container=Default Managed Container)
INFO - Created Ejb(deployment-id=PropertyRegistry, ejb-name=PropertyRegistry, container=Default Singleton Container)
INFO - Created Ejb(deployment-id=ComponentRegistry, ejb-name=ComponentRegistry, container=Default Singleton Container)
INFO - Started Ejb(deployment-id=org.superbiz.registry.ComponentRegistryTest, ejb-name=org.superbiz.registry.ComponentRegistryTest, container=Default Managed Container)
INFO - Started Ejb(deployment-id=PropertyRegistry, ejb-name=PropertyRegistry, container=Default Singleton Container)
INFO - Started Ejb(deployment-id=ComponentRegistry, ejb-name=ComponentRegistry, container=Default Singleton Container)
INFO - Deployed Application(path=/Users/dblevins/examples/simple-singleton)
Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 1.075 sec
Running org.superbiz.registry.PropertiesRegistryTest
INFO - EJBContainer already initialized. Call ejbContainer.close() to allow reinitialization
Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.004 sec
Results :
Tests run: 2, Failures: 0, Errors: 0, Skipped: 0
svn co http://svn.apache.org/repos/asf/openejb/trunk/openejb/examples/simple-singleton cd simple-singleton mvn clean install